1. Field of the Invention
The invention relates to a novel powder coating composition based on epoxy-functional copolymers of olefinically unsaturated monomers and acids or acid derivatives as curing agents, and to the use of these powder coating compositions for the coating of heat-resistant substrates, especially for the production of automotive finishes.
2. Description of the Prior Art
Powder coatings based on epoxy-functional copolymers containing carboxylic acids, especially dicarboxylic acids or dicarboxylic acid anhydrides, as curing agents are known (DE-AS 2,240,312, DE-AS 2,240,314, DE-OS 2,457,826 or U.S. Pat. No. 4,091,048).
Copolymers containing not only epoxy groups but also other functional groups, e.g., hydroxyl, anhydride or amide groups, can also be used for the preparation of powder coatings (cf. e.g. DE-OS 2,441,624, DE-OS 2,441,752, DE-OS 2,441,753, DE-OS 2,457,827, DE-OS 2,457,894, DE-OS 2,509,410, U.S. Pat. No. 3,932,367, U.S. Pat. No. 3,991,132, U.S. Pat. No. 4,374,954).
The powder coatings described in said publications were developed especially for automotive finishes. However, with regard to solvent resistance and chemical resistance, the resulting coatings do not fully satisfy the appropriate practical requirements.
There have been many attempts to improve the inadequate solvent resistance of powder coatings based on epoxy-functional polyacrylates and dicarboxylic acids or their mono- or poly-anhydrides. EP-A-0,299,420 describes polyanhydride crosslinking agents based on aliphatic dicarboxylic acids having 3 to 20 carbon atoms, modified with diols or polyols. Corresponding powder coating compositions with epoxy-functional polyacrylates as binders produce coatings which have good flow properties but which do not possess the necessary resistance to chemicals at the low stoving temperatures of about 140.degree. C. demanded in practice.
EP-A-0,509,392 and EP-A-0,509,393 describe powder coating compositions based on epoxy-functional copolymers and aliphatic or cycloaliphatic carboxylic acids, their anhydrides or polyol-modified anhydrides of dibasic acids. An essential feature of both these inventions is the low stoving temperature of 120.degree. C. which can be achieved. It is disclosed that this is attributable in the case of EP-A-0,509,392 to a content of tert-butyl acrylate or tert-butyl methacrylate of 5 to 50% by weight, and in the case of EP-A-0,509,393 to a content of styrene of 35 to 50% by weight in the epoxy-functional copolymer.
U.S. Pat. No. 4,346,144 describes powder coating compositions containing A) epoxy-functional copolymers, B) aliphatic dicarboxylic acids having 8 to 18 carbon atoms as crosslinking agents, and C) an additional crosslinking component capable of reacting with hydroxyl or carboxyl groups. Examples are alkylated melamine-formaldehyde resins, alkylated glycoluril resins, aliphatic glycidyl ethers and cycloaliphatic diepoxides. Blocked polyisocyanates are also mentioned as additional crosslinking agents in the text and in several examples.
For reasons of environmental protection, however, it is desirable to prepare coatings which are completely free of emissions, regardless of whether they are solvents or decomposition products, such as blocking agents.
Therefore, it is an object of the present invention to provide a novel powder coating composition which does not suffer from said disadvantages of the state of the art, i.e., a composition which can be cured at temperatures below 160.degree. C. to give a smooth-flowing, elastic, high-gloss film resistant to solvents and chemicals.
This object may be achieved by the preparation of the powder coating compositions according to the invention, which are described in greater detail below. The novel powder coating compositions are characterized by the presence of polyaddition products containing uretdione groups as an additional curing component.
The realization that the object of the invention can be achieved by the use of crosslinking components containing uretdione groups can be regarded as surprising because it has previously been recommended that powder coating compositions containing curing agents having uretdione groups should be cured at temperatures above 160.degree. C., preferably above 170.degree. C. (cf. the examples of EP-A-0,045,994 and EP-A-0,045,998).